The immune system is the body's protective network that is designed to, among other things, fend off invasion from pathogens such as bacteria, viruses, chemicals, and even to act as a surveillance system against the development of cancer. Under normal circumstances, the immune system is highly efficient, providing multiple defenses to keep the body healthy.
Stress can weaken the immune system and can produce symptoms such as fever, weight loss, musculoskeletal pain, and fatigue. Functions of the immune system suspected to be impaired under stressed conditions include those of the B-lymphocytes (B cells) and T-lymphocytes (T cells), cytokines, as well as those of the phagocytic and complement systems. Chronic stress can raise cortisol levels, which can further weaken the immune system.